1. Field of the Invention
The present invention relates to an electret capacitor microphone and a method for producing the same.
2. Background Art
Generally, in a process for producing an electret capacitor microphone, for example, as described in Japanese Patent Laid-Open No. 2000-32596, a diaphragm is stretched on and fixed to a diaphragm support ring to produce a diaphragm sub-assembly. A housing is then packed with the diaphragm sub-assembly together with other parts such as a back plate.
As described in this official gazette, a diaphragm made of a thermoplastic resin such as PET (polyethylene terephthalate) is frequently used as the diaphragm in the electret capacitor microphone. A thermoplastic resin film is stretched with predetermined tension and then bonded to a diaphragm support ring.
In the related-art electret capacitor microphone and the method for producing the same, there is however the following problem.
If too large tension is applied on the thermoplastic resin film when the diaphragm is fixed to the diaphragm support ring, the stiffness of the diaphragm becomes too high to sufficiently increase microphone sensitivity. Particularly it is difficult to increase microphone sensitivity in a small-size electret capacitor microphone because the size of the diaphragm is so small that the stiffness of the diaphragm becomes very high even in the case where the applied tension is not changed.
On the other hand, if the tension at the time of stretch and fixation of the diaphragm is set to a small value, the stiffness of the diaphragm can be reduced. The diaphragm is, however, easily crinkled if the tension becomes too small. For this reason, acoustic characteristic of the microphone becomes unstable. Particularly because the supply of the thermoplastic resin film is generally performed by feeding the film out of a film roll, the diaphragm is easily crinkled even in the case where the tension is slightly reduced.